The present invention relates to a wireless communication technique and, more particularly, to a control technique for a reception diversity system which obtains desired reception data by performing switching control of a plurality of systems which individually demodulate reception waves.
A wireless terminal such as a cellular phone terminal receives, as radio waves transmitted from a wireless apparatus such as a base station, not only direct waves which are directly sent from the base station to the wireless terminal but also reflected waves which are sent to the wireless terminal upon being reflected by obstacles such as buildings.
In this case, since a reflected wave differs from a direct wave in propagation distance, even if they are radio waves transmitted from the base station at the same timing, they are received by the wireless terminal at different timings. As a consequence, these radio waves interfere with each other to cause a phenomenon in which radio waves received by the wireless terminal repeatedly vary in strength in predetermined cycles, i.e., fading.
In order to suppress the influence of such fading, a reception diversity system has been widely used and incorporated in a wireless terminal.
FIG. 17 shows an example of the arrangement of a conventional reception diversity system. This reception diversity system includes an RF unit 11, analog signal processing unit 12, digital signal processing unit 13, control unit 14, storage unit 15, and power supply unit 16.
The RF unit 11 demodulates reception waves from the respective antennas, for each of systems 1 to N, and outputs analog reception signals. The analog signal processing unit 12 A/D-converts the analog reception signals from the RF unit 11, and outputs the resultant signals as digital reception signals for each of systems 1 to N. The digital signal processing unit 13 decodes and combines the digital reception signals from the analog signal processing unit 12 for each of systems 1 to N, and outputs the resultant information as reception information. The digital signal processing unit 13 also adjusts the reception gain of the RF unit 11 for each of systems 1 to N on the basis of the result of level calculation performed for each systems 1 to N.
The control unit 14 issues operation instructions to the RF unit 11, analog signal processing unit 12, and digital signal processing unit 13 for each of systems 1 to N on the basis of the reception information from the digital signal processing unit 13. The storage unit 15 stores various kinds of information necessary for control processing by the control unit 14. The power supply unit 16 supplies power necessary for the operations of the RF unit 11, analog signal processing unit 12, digital signal processing unit 13, control unit 14, and storage unit 15.
As described above, in the conventional reception diversity system, the RF unit 11, analog signal processing unit 12, and digital signal processing unit 13 each are provided in parallel with a plurality of systems 1 to N which process reception waves. In this arrangement, a plurality of reception signals obtained from systems 1 to N are combined or switched to improve the reception quality of reception waves, e.g., SNR (Signal to Noise Ratio) or SIR (Signal to Interference Ratio) or suppress variations in the level of reception waves to stabilize them, thereby suppressing the influence of fading.
As shown in FIG. 18, in such a reception diversity system, for example, during standby time, systems 1 to N are made to intermittently perform reception operation in synchronism with each other to adjust the reception gain of each system, thereby improving the overall reception quality obtained by combining reception signals from the respective systems. Consequently, power consumption increases with an increase in the number of systems. If, therefore, a conventional reception diversity system is incorporated in a wireless terminal powered by a secondary battery, e.g., a cellular phone terminal, the consumption of the secondary battery is higher than that of a terminal without a reception diversity system. As a consequence, the period in which the wireless terminal can be used by the secondary battery, e.g., a speech communication time and standby time, is greatly reduced.
In order to suppress such power consumption, in a conventional reception diversity system, there is conceivable a method of operating only one of a plurality systems while stopping the remaining systems in each intermittent reception period in intermittent reception operation, e.g., a standby time, and stopping all the systems in an intermittent stop period.
In a conventional reception diversity system, however, if only one of a plurality of systems is operated while the remaining systems are stopped in each intermittent operation period, the essential function required for the reception diversity system cannot be effectively used.
In a conventional reception diversity system, when, for example, the operation of a system which is not selected is simply stopped, the system cannot grasp variations in reception level during an intermittent stop period, and hence uses a reception gain which is permanently determined in advance at the time of resumption of reception operation in the next intermittent operation period. This makes it impossible to properly adjust the reception gain while following up the above variations in reception level.
In addition, in a conventional reception diversity system, when only one of the systems is operated in an intermittent operation period while the remaining systems are stopped, total reception quality such as an SNR or SIR value obtained when all the systems are concurrently operated, and the resultant reception signals are combined cannot be grasped. This makes it impossible to properly perform reception control such as cell switching determination in a wireless telephone system, and hence it is impossible to obtain merits derived from the reception diversity system, e.g., an improvement in reception quality.